1. Field of the Invention.
This invention relates in general to the operation of a magnetic storage device, and more particularly to method and apparatus for optimizing flying height control using heated sliders.
2. Description of Related Art.
Modem computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. In such devices, read-write heads are used to write data on or read data from an adjacently rotating hard or flexible disk.
A head/disk assembly typically includes one or more commonly driven magnetic data storage disks rotatable about a common spindle. At least one head actuator moves one or more magnetic read/write heads radially relative to the disks to provide for reading and/or writing of data on selected circular concentric tracks of the disks. Each magnetic head is suspended in close proximity to one of the recording disks and supported by an air bearing slider mounted to the flexible suspension. The suspension, in turn, is attached to a positioning actuator.
During normal operation, relative motion between the head and the recording medium is provided by the disk rotation as the actuator dynamically positions the head over a desired track.
The relative motion provides airflow along the surface of the slider facing the medium, creating a lifting force. The lifting force is counterbalanced by a known suspension load so that the slider is supported on a cushion of air. Airflow enters the leading edge of the slider and exits from the trailing end. The head normally resides toward the trailing end, which tends to fly closer to the recording surface than the leading edge.
Existing magnetic storage systems use magnetoresistive (MR) heads to read data from magnetic media and to uses inductive heads to write data onto magnetic media. MR disk drives use a rotatable disk with concentric data tracks containing the user data, a read/write head that may include an inductive write head and an MR read head for writing and reading data on the various tracks, a data readback and detection channel coupled to the MR head for processing the data magnetically recorded on the disk, an actuator connected to a carrier for the head for moving the head to the desired data track and maintaining it over the track centerline during read or write operations.
There is typically a plurality of disks stacked on a hub that is rotated by a disk drive spindle motor. A housing supports the drive motor and head actuator and surrounds the head and disk to provide a substantially sealed environment for the head-disk interface. The head carrier is typically an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained in very close proximity to the disk surface by a suspension that connects the slider to the actuator. The spacing between the slider and the disk surface is called the flying height and its precise value is critical to the proper function of the reading and writing processes.
The inductive write head and MR read head are patterned on the trailing end of the slider, which is the portion of the slider that flies closest to the disk surface. The slider is either biased toward the disk surface by a small spring force from the suspension, and/or is “self-loaded” to the disk surface by means of a “negative-pressure” air-bearing surface on the slider.
The MR sensor detects magnetic field signals through the resistance changes of a magnetoresistive element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in an MR read head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage. The conventional MR sensor used in magnetic recording systems operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is “pinned”, and thus prevented from rotating in the presence of an external magnetic field. This type of MR sensor is called a “spin valve” sensor.
The read-write heads have been designed so that they will fly over the surface of the rotating disk at a very small, and relatively constant distance above the disk. The separation between the read-write head and the disk is called the flying height, and is maintained by a film of air. The flying height is critical to proper function during reading and writing. If the flying height is too high during read, the read head will not be able to resolve the fine detail of the magnetic signal, thereby resulting in undecipherable data. Similarly, if the flying height is too high during a write, the magnetic flux lines that intersect the plane of the disk surface become weaker, thereby leading to loss of resolution.
As magnetic recording areal density increases, the flying height between the head and the disk continues to shrink. As discrete data storage areas are placed more closely to one another, the transducer must be positioned more closely to the recording surface to distinguish between adjacent storage areas. In recent year, head flying heights have been decreased largely due to improved techniques for reducing media surface roughness. Further reductions in flying height are enabled by a super smooth polishing of media surfaces in data recording areas while also providing an adjacent head contact zone, textured to avoid stiction problems.
Flying height flying height control itself also has two distinct but related aspects: 1) achieving a desired low flying heightflying height during disk rotation and 2) keeping the flying heightflying height as close as possible to a constant during disk rotation. The first aspect relates to the capability to achieve a sustainable average low flying heightflying height while the second aspect relates to the stability of the flying height during disk rotation regardless of the average flying heightflying height. These two aspects are further explained as follows.
Increasingly higher areal density in disk drives requires that, in addition to having direct impact on radial positioning resolution, the flying heightflying height be decreased in order to obtain higher signal resolution. That is, there is a pressing need for the air-bearing surface of a slider to fly as close to the media as possible, without touching the media to produce better resolution of data on the media, because read/write signal amplitude is dependent on the distance between the magnetic medium and the read/write head, and close spacing drastically improves transducer performance without having to improve sensitivity of the transducer.
The schemes used in conventional hard drives to lower the flying height primarily address air bearing designs and smoothening of the disk surface. However, this approach is reaching the limit of its ability to meet the ever decreasing flying height requirement and the accompanying need to control disturbances present in the hard drive environment. Other various methods have been used for controlling transducer head flying height. For example, the head-media spacing loss due to thermal expansion of the transducer may be addressed by optimizing the thermal mechanical structure and properties of the transducer. Such a method is in essence a passive countermeasure and fails to actively adjust the pole tip position of the transducer to consistently minimize its impact on head-media spacing.
Also, a transducer is movable toward and away from the air bearing surface in response to changes in the slider operating temperature. The transducer movement is either due to a difference in thermal expansion coefficients between a transducing region of the slider incorporating the transducer and the remainder of the slider body, or by virtue of a strip of thermally expansive material incorporated into the slider near the transducer to contribute to the displacement by its own expansion.
In this regard, a flying height control device has been proposed that include a resistance heating element mounted to the slider body. The heating element is disposed within a transducing region substantially encapsulated in the slider body. The slider body and transducing region have different thermal expansion coefficients. Thus, the position of the transducer may be determined primarily by the differences in expansion, as the slider is heated. Alternatively, the heating element may be formed using a thermally expansive and electrically conductive material mounted to the slider body near the transducer. In this arrangement, the heating element provides a thermal expansion region with a higher thermal expansion coefficient than the slider body. The material thermally expands when subject to a bias current and elastically expands adjacent material, thus to play a direct role in determining transducer position.
In such slider designs, the main function of the heater is to heat the area to cause mechanical deformation altering the flying height of the slider. The degree of mechanical deformation and the time required to achieve the deformation depends on many factors, but the location of the heater with respect to the ABS surface is perhaps the most important one. Typically, the desired time constant is on the order of 100–200 μsec, which requires the heater to be placed near the ABS. However, as the heater is moved closer to the ABS surface, the heat generation from the heater also causes MR temperature to increase. This rise in the MR temperature can decrease the reliability of the MR head.
The fast response time (100–200 μsec), however, is only needed for what is called a first sector write. The time constant for the write head is on the order of 100–200 μsec. This means that when the write head begins to write, the magnetic spacing can acutally be higher for the first 100–200 μsec. Hence, during the initial 100–200 μsec, the data may not be written correctly since the magnetic spacing is too large. To overcome this problem, heater is used to heat the writer region to cause similar deformation immediately before writing takes place.
The heater is also used to compensate for the same deformation during reading, to offset the flying height sigmas, and to adjust for low temperature conditions. These three cases, however, does not requires such a fast time constant and thus the heater does not need to be close to the ABS surface. It can be placed sufficiently away from the ABS surface so that the temperature rise to the MR head is minimal.
Nevertheless, achievement of all of these objectives is difficult since the demands and requirements are different.
It can be seen then that there is a need for a method and apparatus for optimizing flying height control using heated sliders.